Nowadays, polymer materials are frequently used for pipes for various purposes, such as fluid transport, i.e. transport of liquid or gas, e.g. water or natural gas, during which the fluid can be pressurised. Moreover, the transported fluid may have varying temperatures, usually within the temperature range from about 0° C. to about 100° C. Such pipes are preferably made of polyolefin plastic, usually unimodal polyethylene such as medium density polyethylene (MDPE; density: 0.930-0.942 g/cm3) and high density polyethylene (HDPE; density: 0.945-0.965 g/cm3).
According to WO 00/01765 a polymer composition intended for pressurised pipes for the transport of gases and liquids such as cold water is known. The composition comprises a multimodal polyethylene with a density of 0.930-0.965 g/cm3, an MFR5 of 0.2-1.2 g/10 min, an Mn of 8000-15000, an Mw of 180−330×103, and an Mw/Mn of 20-35. The multimodal polyethylene further comprises a low molecular weight (LMW) ethylene homopolymer fraction and a high molecular weight (HMW) ethylene copolymer fraction, said HMW fraction having a lower molecular weight limit of 3500, and a weight ratio of the LMW fraction to the HMW fraction of (35-55):(65:45).
According to WO 03/033586 a polymer pipe for hot fluids (temperature at least 60° C., usually 60-100° C., such as 70-90° C.) is known. The polymer pipe is characterised in that it comprises a multimodal polyethylene with a high molecular weight (HMW) fraction and a low molecular weight (LMW) fraction where said HMW fraction has a density of at least 0.920 g/cm3, and that the multimodal polyethylene has a time to failure at 95° C. and 3.6 MPa of at least 165 h determined according to DIN 16 833 and a modulus of elasticity of at most 900 MPa determined according to ISO 527-2/1B.
Crosslinking improves e.g. heat deformation resistance of polyethylene and therefore pipes for hot water applications, such as pipes for floor heating, or for hot water distribution are usually made of crosslinked polyethylene (PEX). However, prior art pipes such as pipes of crosslinked unimodal high density polyethylene (HDPE-X) have several drawbacks. Thus, in order to meet the high demands of the so-called HDPE-X norm for hot and cold water applications (e.g. DIN 16 892/prEN ISO 12318) it is necessary to use polyethylene of a relatively high density. This entails that the resulting pipe is relatively stiff. This stiffness becomes even more pronounced when barrier layers are applied on top of or within the core pipe.
In order to improve the crosslinking response and hence reduce the consumption of crosslinking agent, e.g. peroxide, when crosslinking pipes of polyethylene, it is generally desired to use an ethylene polymer of relatively low melt flow rate (MFR), i.e. high molecular weight. However, this results in the drawback of poor processability, i.e. a reduced line speed at extrusion.
Another problem that may arise is insufficient melt strength when using polymers having a higher MFR in order to achieve better extrudability. In a pipe manufacturing system, where pipes are extruded and crosslinked in a separate system, melt strength is required to keep dimensional stability of the partly molten, uncrosslinked pipe until it becomes crosslinked. In the worst case, lack of melt strength means that it may not be possible to prepare a pipe from the polymer, because the polymer parison collapses when it exits from the extruder. Higher MFR resins also have poorer crosslinkability, which results in that a larger amount of crosslinking agent or a stronger irradiation dose must be used.
It is an object of the present invention to eliminate or alleviate the above mentioned problems of the prior art and provide a polymer composition with improved crosslinking response and flexibility and with good processability into a pipe.